AC Disconnect of Power Over Ethernet Devices

ABSTRACT

Embodiments of power sourcing equipment (PSE) utilizing AC disconnect are provided herein. In one embodiment, a PSE is provided that includes a DC supply configured to provide a DC voltage over a data communications medium, a controller configured to provide an AC disconnect signal over the data communications medium, and a parallel inductor-capacitor (LC) circuit coupled between the DC supply and the data communications medium. The parallel LC circuit is configured to isolate the DC supply from the AC disconnect signal. In another embodiment, a PSE is provided that includes a DC supply configured to provide a DC voltage at an output, an inductor coupled between the output of the DC supply and a data communications medium, and a capacitor coupled between the data communications medium and ground. The inductor and capacitor form a series LC circuit configured to generate an AC disconnect signal.

FIELD OF THE INVENTION

This application relates generally to Power over Ethernet (PoE) devicesand, more specifically, to apparatuses for AC disconnect of PoE devices.

BACKGROUND

The IEEE 802.3af and 802.3at specifications, also known as Power overEthernet (PoE), provides a framework for delivering DC powerconcurrently with data over standard Ethernet cabling. A PoE systemincludes three basic components: power sourcing equipment (PSE) forproviding power, a powered device (PD) for receiving and consuming thepower, and cabling for transferring the power from the PSE to the PD.The PSE, as defined by the IEEE 802.3af/t specifications, performs muchof the basic power provisioning process, including detection,classification, operation, and disconnection.

Detection is first performed by the PSE to determine if a valid PD isconnected to its power providing output. Detection is carried out byinducing a small voltage at the output of the PSE to detect a specific25 KΩ signature resistor. This signature indicates that a valid PD isconnected and that the provision of power to the PD can begin.

After a valid PD is detected, an optional classification stage can beperformed to estimate the amount of power required by the PD. To performclassification, the PSE again induces a voltage around 15.5-20.5 Vdc fora period of time within 10 to 75 ms. The current consumed by the PDduring this time period indicates to the PSE its power classification.

Following detection and optional classification, the output power of thePSE can be increased, during the operation stage, to its full voltagecapacity, which is typically around 48 Vdc. The output voltage of thePSE is gradually increased to its full voltage capacity to prevent highfrequency noise from disrupting data being transferred concurrently withthe power.

The final stage of the power provisioning process involves removal ofpower following the disconnection of the PD connected to the PSE. TheIEEE 802.3af/t specifications define two specific techniques for powerdisconnection; namely, DC disconnect and AC disconnect. Both methodsprovide the same desired result—the detection of a disconnected PD andthe removal of power within 300 to 400 ms thereafter. The removal ofpower when a PD is disconnected is important because the PD may bereplaced by a non-PoE-ready device, which may result in damage.

DC disconnect is performed at the PSE by measuring the current consumedby the PD. If the PD is disconnected at any point, the consumption ofcurrent by the PD would cease, indicating disconnection of the PD. ACdisconnect, on the other hand, entails the addition of a low AC signalon top of the 48 Vdc operating voltage. The returned AC signal amplitudeis monitored at the PSE. While the PD is connected, the low impedance ofthe PD lowers the returned AC signal. During disconnection, however, theAC signal level will increase, indicating disconnection of the PD.

In conventional PSEs implementing AC disconnect, a diode is used toisolate the DC source, providing the 48 Vdc operating voltage, from theAC disconnect signal. Depending on the type of diode utilized, the diodecan have a forward voltage drop of 0.3-0.7 Vdc at 600 mA, or a totalpower consumption around 0.2-0.4 W, for example. Not only does theisolation diode increase overall power consumption, but furtherincreases the overall temperature at the media dependent interface (MDI)of the PSE. This excess power consumption and temperature becomes evenmore apparent in multi-port hubs or switches that are PoE-ready. Forexample, in a 24-port hub that is PoE-ready, 24 separate diodes (one foreach port) can be required to isolate the DC supply from the ACdisconnect signal(s).

Therefore, what is needed is an apparatus for isolating a DC supply of aPSE from an AC disconnect signal, while limiting any additional powerconsumption and heat produced as a result thereof.

BRIEF DESCRIPTION OF THE DRAWINGS/FIGURES

The accompanying drawings, which are incorporated herein and form a partof the specification, illustrate the present invention and, togetherwith the description, further serve to explain the principles of theinvention and to enable a person skilled in the pertinent art to makeand use the invention.

FIG. 1 illustrates an exemplary PoE system in which embodiments of thepresent invention can be implemented.

FIG. 2 illustrates portions of an exemplary PSE, utilizing aconventional isolation technique.

FIG. 3 illustrates portions of an exemplary PSE, utilizing a parallelinductor-capacitor (LC) circuit, according to embodiments of the presentinvention.

FIG. 4 illustrates portions of an exemplary PSE, utilizing a seriesinductor-capacitor (LC) circuit, according to embodiments of the presentinvention.

The present invention will be described with reference to theaccompanying drawings. The drawing in which an element first appears istypically indicated by the leftmost digit(s) in the correspondingreference number.

DETAILED DESCRIPTION

In the following description, numerous specific details are set forth inorder to provide a thorough understanding of the invention. However, itwill be apparent to those skilled in the art that the invention,including structures, systems, and methods, may be practiced withoutthese specific details. The description and representation herein arethe common means used by those experienced or skilled in the art to mosteffectively convey the substance of their work to others skilled in theart. In other instances, well-known methods, procedures, components, andcircuitry have not been described in detail to avoid unnecessarilyobscuring aspects of the invention.

References in the specification to “one embodiment,” “an embodiment,”“an example embodiment,” etc., indicate that the embodiment describedmay include a particular feature, structure, or characteristic, butevery embodiment may not necessarily include the particular feature,structure, or characteristic. Moreover, such phrases are not necessarilyreferring to the same embodiment. Further, when a particular feature,structure, or characteristic is described in connection with anembodiment, it is submitted that it is within the knowledge of oneskilled in the art to effect such feature, structure, or characteristicin connection with other embodiments whether or not explicitlydescribed.

1. Operating Environment

FIG. 1 provides a diagram of an exemplary PoE system 100 in whichembodiments of the present invention can be implemented. PoE system 100includes a switch/hub 102 and a powered end station 104. Powered endstation 104 can be any one of several different network nodes; forexample, powered end station 104 can be an internet protocol (IP) phoneor a wireless access point, to name a few. Switch/hub 102 provides powerto powered end station 104 over conductor pairs 106 and 108, which arefurther configured to carry differential data between switch/hub 102 andpowered end station 104.

As illustrated in FIG. 1, switch/hub 102 includes a transceiver 110 thatachieves full duplex transmit and receive capability using adifferential transmit port (TX) 112 and a differential receive port (RX)114. A first transformer 116 couples high speed data between transmitport 112 and conductor pair 106. Likewise, a second transformer 118couples high speed data between receive port 114 and conductor pair 108.The respective transformers 116 and 118 pass the high speed data to andfrom transceiver 110, but isolate any low frequency or DC voltage fromthe transceiver ports, which may be sensitive to large magnitudevoltages.

Transformer 116 includes primary and secondary windings, where thesecondary winding (coupled to conductor pair 106) includes a center tap120. Transformer 118 includes primary and secondary windings, where thesecondary winding (coupled to conductor pair 108) includes a center tap122. Power Sourcing Equipment (PSE) 124 generates an output voltage thatis applied across center taps 120 and 122 of transformers 116 and 118 onthe conductor pair sides of the transformers. Center tap 120 is coupledto a first output of PSE 124, and center tap 122 is coupled to a secondoutput of PSE 124. As such, transformers 116 and 118 isolate the DCvoltage provided by PSE 124 from sensitive data ports 112 and 114 oftransceiver 110. An example DC output voltage provided by PSE 124 issubstantially 48 volts, but other voltages could be used depending onthe voltage/power requirements of powered end station 104.

PSE 124 includes a PSE controller (not shown) that controls theprovisioning of DC power to powered end station 104. More specifically,the PSE controller of PSE 124 performs the basic power provisioningprocess defined by the IEEE 802.3af/t specifications, includingdetection, classification (optionally performed), operation, anddisconnection. In an embodiment, the PSE controller of PSE 124 utilizesan AC disconnect technique to discontinue the supply of DC power overconductor pairs 106 and 108 when powered end station 104 isdisconnected.

Still referring to FIG. 1, the contents and functionality of powered endstation 104 will now be discussed. Powered end station 104 includes atransceiver 126 having full duplex transmit and receive capability thatis achieved using differential receive port (RX) 128 and differentialtransmit port (TX) 130. A transformer 132 couples high speed databetween conductor pair 106 and receive port 128. Likewise, a transformer134 couples high speed data between transmit port 130 and conductor pair108. Transformers 132 and 134 pass the high speed data to and fromtransceiver 126, but isolate any low frequency or DC voltage fromsensitive data ports 128 and 130.

Transformer 132 includes primary and secondary windings, where thesecondary winding (coupled to conductor pair 106) includes a center tap136. Likewise, transformer 134 includes primary and secondary windings,where the secondary winding (coupled to conductor pair 108) includes acenter tap 138. Center taps 136 and 138 supply the power carried overconductor pairs 106 and 108 to a powered device (PD) 140.

PD 140 can include a PD controller (not shown) to monitor the voltageand current provided to it. The PD controller can further provide thenecessary impedance signatures on the return conductor 108 duringdetection, so that the PSE controller, implemented within PSE 124, canrecognize PD 140 as a valid PoE-ready device.

During operation, a direct current (I_(DC)) flows from PSE 124 throughcenter tap 120 and divides into a first current (I₁) and a secondcurrent (I₂) that are carried over conductor pair 106. The first current(I₁) and the second current (I₂) recombine at center tap 136 to reformthe direct current (I_(DC)) used to power PD 140. On return, the directcurrent (I_(DC)) flows from PD 140 through center tap 138, and dividesfor transport over conductor pair 108. The return DC current recombinesat center tap 122 and returns to PSE 124.

As discussed above, data transmission between switch/hub 102 and poweredend station 104 can occur concurrently with the provisioning of DC powerby PSE 124. Data is carried differentially over conductor pairs 106 and108 between switch/hub 102 and powered end station 106. Because data iscarried differentially over conductor pairs 106 and 108, the data isideally unaffected by the DC power transfer, which appears as commonmode.

It should be noted that other alternative configurations for PoE system100 can be used without departing from the scope and spirit of thepresent invention. For example, in an alternative configuration of PoEsystem 100, DC power, supplied by PSE 124, is transmitted over the sparewire pairs of the Ethernet cabling as specified by the IEEE 802.3af/tEthernet specifications.

2. Conventional Apparatus for Isolation

FIG. 2 illustrates portions of an exemplary PSE 124 a, utilizing aconventional isolation technique. PSE 124 a includes a PSE controller200 and a DC supply 202. In an embodiment, DC supply 202 is configuredto provide 48 Vdc across nodes 120 and 122, which are coupled to PD 140as illustrated in FIG. 1. PSE controller 200 is configured to controlPSE 124 a to perform the basic power provisioning process defined by theIEEE 802.3af/t specifications, including detection, classification(optionally performed), operation, and disconnection.

Detection is first performed by PSE controller 200 to determine if avalid PD is coupled to output nodes 120 and 122. Detection isspecifically carried out by producing a small voltage across nodes 120and 122 to detect a specific signature resistor, such as 25 KΩ. Thissignature indicates that a valid PD, such as PD 140 illustrated in FIG.1, is coupled to PSE 124 a and that the provision of power to the PD canbegin.

After a valid PD is detected, an optional classification stage can beperformed to estimate the amount of power required by the PD. To performclassification, PSE controller 200 again produces a voltage (e.g.,around 15.5-20.5 Vdc) for a predetermined period of time (e.g., 10 to 75ms). The current consumed by the PD during this predetermined period oftime indicates to PSE controller 200 the power classification of the PD.

Following detection and optional classification, the output power of PSE124 a can be increased, during the operation stage, to its full voltagecapacity, which is typically around 48 Vdc. The output voltage of thePSE is gradually increased to its full voltage capacity to prevent highfrequency noise from disrupting data being transferred concurrently withthe power.

It should be noted that portions of the entire structure for performingdetection, classification, and operation (i.e., the first three stagesof the basic power provisioning process defined by the IEEE 802.3af/tspecifications) have been omitted from the illustration in FIG. 2 forthe sake of clarity.

The final stage of the power provisioning process involves removal ofpower following the disconnection of the PD inductively coupled to PSE124 a at nodes 120 and 122. The IEEE 802.3af/t specifications define twospecific techniques for power disconnection; namely, DC disconnect andAC disconnect. Both methods provide the same desired result—thedetection of a disconnected PD and the removal of power within 300 to400 ms thereafter. The removal of power when a PD is disconnected isimportant because the PD may be replaced by a non-PoE-ready device,which may result in damage of the device.

PSE 124 a is configured to perform AC disconnect, which entails theaddition of a low AC signal on top of the 48 Vdc operating voltageprovided by DC supply 202. To generate the low AC signal, PSE controller200 includes a charge pump 204 that is coupled at an input to the 48 Vdcsignal produced by DC supply 202. Charge pump 204 is configured togenerate a voltage signal that is higher than the 48 Vdc signal receivedat its input. In embodiment, the voltage signal produced by charge pump204 is substantially 3 V higher than the 48 Vdc signal; that is, chargepump 204 produces a 51 Vdc signal that is applied across charge pumpcapacitor C_(CP). Capacitor C_(CP) can be utilized at the output ofcharge pump 204 to smooth variations in the 51 Vdc signal produced.

Switch S1, further included in PSE controller 200 and coupled to the 51Vdc signal, can be switched on and off to produce the AC disconnectsignal, which transitions back and forth from 48 V to 51 V at thefrequency in which switch S1 is switched on and off. In an embodiment,switch S1 is controlled by a control signal 206 provided by anoscillator (not shown). In a further embodiment, switch S1 is switchedon and off at a frequency of 27 Hz. The resulting AC disconnect signal(of frequency 27 Hz) is coupled to node 120 through resistor R₁ anddiode D₁, where diode D₁ provides reverse isolation.

After generation and provisioning of the AC disconnect signal, thereturned AC signal amplitude is monitored at PSE 124 a. While the PD isconnected, the low AC impedance of the PD lowers the returned AC signal.During disconnection, however, the AC impedance across terminals 120 and122 increases significantly and, as a result, the AC signal level willincrease, indicating disconnection of the PD. Switch S₂ is controlled bycontrol signal 208 to discontinue the provision of DC power whendisconnection is detected. Specifically, to discontinue the provision ofDC power, switch S₂ is opened. It should be noted that the structureused to monitor the returned AC signal has been omitted from FIG. 2 forthe sake of clarity.

PSE 124A further implements a conventional technique to isolate thegenerated AC disconnect signal from the DC supply 202. Specifically, PSE124A utilizes a diode D₂ to isolate DC supply 202, which provides the 48Vdc operating voltage across nodes 120 and 122, from the AC disconnectsignal. Depending on the type of diode utilized, the diode can have aforward voltage drop of 0.3-0.7 Vdc at 600 mA, or a total powerconsumption around 0.2-0.4 W, for example. Not only does diode D₂increase overall power consumption, but further increases the overalltemperature at the media dependent interface (MDI) of PSE 124 a. Thisexcess power consumption and temperature becomes even more apparent andprohibitive in multi-port hubs or switches that are PoE-ready. Forexample, in a 24-port hub that is PoE-ready, 24 separate diodes (one foreach port) can be required to isolate the DC supply from the ACdisconnect signal.

Therefore, what is needed is an apparatus for isolating DC supply 202from the AC disconnect signal, while limiting additional powerconsumption and heat produced as a result thereof.

It should be noted that protection capacitor C_(P) and protection diodeD_(P), further illustrated in FIG. 2, can be used to neutralize surgeevents on and across nodes 120 and 122. In an embodiment, diode DP is atransient voltage suppression (TVS) diode used to limit the differentialvoltage across nodes 120 and 122.

3. Parallel Inductor-Capacitor (LC) Circuit

FIG. 3 illustrates portions of an exemplary PSE 124 b, according toembodiments of the present invention. The implementation of PSE 124 beliminates the need for diode D₂ illustrated in FIG. 2 and used toisolate DC supply 202 from the generated AC disconnect signal. Diode D₂has been replaced by a parallel combination of an inductor L₁ and acapacitor C₁ in the implementation of PSE 124 b illustrated in FIG. 3.

The parallel inductor-capacitor (LC) circuit 300, formed from inductorL₁ and capacitor C₁, has a resonant frequency given by:

$f = \frac{1}{2\; \pi \sqrt{L_{1}C_{1}}}$

At resonance, the effective impedance of parallel LC circuit 300 isextremely large; in fact, the theoretical impedance is infinite. Thus,the value of L₁ (specified in Henries) and the value of C₁ (specified inFarads) can be selected such that their parallel combination has aresonant frequency equal to the fundamental frequency of the ACdisconnect signal generated by PSE controller 200. For example, assumingthat the AC disconnect signal generated by PSE controller 200 has afundamental frequency of 4 kHz, a value of 680 μH for inductor L₁ and2.2 μF for capacitor C₁ establishes a resonant frequency ofapproximately 4 kHz for the parallel LC combination. Therefore, thisspecific implementation of the LC combination, formed by inductor L₁ andcapacitor C₁, will present an extremely large impedance to the 4 kHz ACdisconnect signal and effectively isolate DC supply 202 from the 4 kHzAC disconnect signal.

Moreover, the power consumed by inductor L₁ and capacitor C₁ isconsiderably less than diode D₂, illustrated in FIG. 2. Inductor L1 andcapacitor C1 will generally only dissipate power as a result of theirparasitic resistances, which are typically fairly low valued, especiallycompared to the effective resistance of a forward biased diode.

It should be noted that parallel LC circuit 300 can include additionalactive and passive components and is no way limited to the structureillustrated in FIG. 3. For example, parallel LC circuit 300 can furtherinclude a resistor or additional inductive and capacitive components.

4. Series Inductor-Capacitor (LC) Circuit

FIG. 4 illustrates portions of an exemplary PSE 124 c, according toembodiments of the present invention. The implementation of PSE 124 celiminates the need for charge pump 204, capacitor C_(CP), switch S₁,resistor R₁, and diode D₁, all of which were used, at least in part, togenerate the AC disconnect signal as illustrated in FIG. 2. In addition,diode D2, used in FIG. 2 to isolate DC supply 202 from the generated ACdisconnect signal, has been eliminated and replaced by an inductor L₁.

Inductor L₁ and capacitor C_(P) form a series inductor-capacitor (LC)circuit that produces an oscillation (i.e., an AC disconnect signal)when a PD, coupled to nodes 120 and 122, is disconnected. The occurrenceof this oscillation can be monitored for by PSE controller 200 to detectthe disconnection of the PD and to discontinue the provisioning of DCpower.

Specifically, during operation, DC supply 202 is configured to provide48 Vdc (for example) to the PD inductively coupled to nodes 120 and 122.Current flows through inductor L₁ during operation and out node 120.When the PD is disconnected, inductor L₁ resists changing the currentflowing through it and continues to supply current, which now chargescapacitor C_(P). Eventually, the energy stored in the magnetic field ofinductor L₁ is exhausted and the current supplied by inductor L₁ ceases.However, the charge now stored on capacitor C_(P) will begin to flowback through inductor L1, re-establishing its magnetic field. When allthe charge stored on capacitor C_(P) has been dissipated, energy willagain be extracted from the magnetic field of the inductor to continuethe flow of current.

This flow of charge, back and forth between capacitor C₁ and inductorL₁, following disconnection, produces an oscillation that can bedetected by PSE controller 200 to signal disconnection. Once detectedswitch S₂ can be opened by control signal 208 to stop the provisioningof DC power. The series LC circuit is often referred to as a tankcircuit, which has similar properties to water sloshing back and forthin a tank.

It should be noted that the frequency of oscillation, produced by theseries LC circuit, is specified by its resonant frequency, given by:

$f = \frac{1}{2\pi \sqrt{L_{1}C_{P}}}$

For example, assuming a value of 4.7 μH for inductor L₁ and a value of0.01 μF for capacitor C_(P), the frequency of oscillation produced bythe series LC circuit will be approximately 700 kHz. This 700 kHzoscillation provides an effective AC disconnect signal that can bemonitored for and detected by PSE controller 200 to stop theprovisioning of DC power.

It should be further noted that an additional capacitor, other thanprotection capacitor C_(P) can be utilized to form the series LC circuitillustrated in FIG. 4. In addition, diode D_(P) can be configured toprevent possible over voltages from occurring across nodes 120 and 122as a result of the oscillations produced by the series LC circuitillustrated in FIG. 4.

5. Conclusion

It is to be appreciated that the Detailed Description section, and notthe Abstract section, is intended to be used to interpret the claims.The Abstract section may set forth one or more but not all exemplaryembodiments of the present invention as contemplated by the inventor(s),and thus, are not intended to limit the present invention and theappended claims in any way.

The present invention has been described above with the aid offunctional building blocks illustrating the implementation of specifiedfunctions and relationships thereof. The boundaries of these functionalbuilding blocks have been arbitrarily defined herein for the convenienceof the description. Alternate boundaries can be defined so long as thespecified functions and relationships thereof are appropriatelyperformed.

The foregoing description of the specific embodiments will so fullyreveal the general nature of the invention that others can, by applyingknowledge within the skill of the art, readily modify and/or adapt forvarious applications such specific embodiments, without undueexperimentation, without departing from the general concept of thepresent invention. Therefore, such adaptations and modifications areintended to be within the meaning and range of equivalents of thedisclosed embodiments, based on the teaching and guidance presentedherein. It is to be understood that the phraseology or terminologyherein is for the purpose of description and not of limitation, suchthat the terminology or phraseology of the present specification is tobe interpreted by the skilled artisan in light of the teachings andguidance.

The breadth and scope of the present invention should not be limited byany of the above-described exemplary embodiments, but should be definedonly in accordance with the following claims and their equivalents.

What is claimed is:
 1. Power sourcing equipment (PSE) for providing DCpower to a powered device (PD) over a data communications medium, thePSE comprising: a DC supply configured to provide a DC voltage over thedata communications medium; a controller configured to provide an ACdisconnect signal over the data communications medium; and a parallelinductor-capacitor (LC) circuit coupled between the DC supply and thedata communications medium, the parallel LC circuit configured toisolate the DC supply from the AC disconnect signal.
 2. The PSE of claim1, wherein the parallel LC circuit has an associated resonant frequencythat is substantially equal to a frequency of the AC disconnect signal.3. The PSE of claim 2, wherein the frequency of the AC disconnect signalis a fundamental frequency of the AC disconnect signal.
 4. The PSE ofclaim 1, wherein data is transmitted concurrently with the DC power overthe data communications medium.
 5. The PSE of claim 4, wherein the datais transmitted in accordance with the IEEE 802.3 standard.
 6. The PSE ofclaim 1, wherein the controller further comprises: a charge pump coupledto the DC supply and configured to provided an output voltage greaterthan the DC voltage provided by the DC power supply; and a switchconfigured to intermittently couple the output voltage of the chargepump to the data communications medium to provide the AC disconnectsignal.
 7. Power sourcing equipment (PSE) for providing DC power to apowered device (PD) over spare wires of a data communications medium,the PSE comprising: a DC supply configured to provide a DC voltage overthe spare wires of the data communications medium; a controllerconfigured to provide an AC disconnect signal over the spare wires ofthe data communications medium; and a parallel inductor-capacitor (LC)circuit coupled between the DC supply and the spare wires of the datacommunications medium, the parallel LC circuit configured to isolate theDC supply from the AC disconnect signal.
 8. The PSE of claim 7, whereinthe parallel LC circuit has an associated resonant frequency that issubstantially equal to a frequency of the AC disconnect signal.
 9. ThePSE of claim 8, wherein the frequency of the AC disconnect signal is afundamental frequency of the AC disconnect signal.
 10. The PSE of claim7, wherein data is transmitted in accordance with the IEEE 802.3standard over the data communications medium.
 11. The PSE of claim 7,wherein the controller further comprises: a charge pump coupled to theDC supply and configured to provided an output voltage greater than theDC voltage provided by the DC power supply; and a switch configured tointermittently couple the output voltage of the charge pump to the sparewires of the data communications medium to provide the AC disconnectsignal.
 12. Power sourcing equipment (PSE) for providing DC power to apowered device (PD) over a data communications medium, the PSEcomprising: a DC supply configured to provide a DC voltage at an output;an inductor coupled between the output of the DC supply and the datacommunications medium; and a capacitor coupled between the datacommunications medium and ground, wherein the inductor and capacitorform a series inductor-capacitor (LC) circuit configured to generate anAC disconnect signal.
 13. The PSE of claim 12, wherein the series LCcircuit is configured to generate the AC disconnect signal if the PD isdisconnected from the data communications medium.
 14. The PSE of claim12, wherein the series inductor-capacitor (LC) circuit forms a tankcircuit.
 15. The PSE of claim 12, wherein the AC disconnect signal has afrequency substantially equal to the resonant frequency of the series LCcircuit.
 16. Power sourcing equipment (PSE) for providing DC power to apowered device (PD) over spare wires of a data communications medium,the PSE comprising: a DC supply configured to provide a DC voltage at anoutput; an inductor coupled between the output of the DC supply and thespare wires of the data communications medium; and a capacitor coupledbetween the spare wires of the data communications medium and ground,wherein the inductor and capacitor form a series inductor-capacitor (LC)circuit configured to generate an AC disconnect signal.
 17. The PSE ofclaim 16, wherein the series LC circuit is configured to generate the ACdisconnect signal if the PD is disconnected from the data communicationsmedium.
 18. The PSE of claim 16, wherein the series inductor-capacitor(LC) circuit forms a tank circuit.
 19. The PSE of claim 16, wherein theAC disconnect signal has a frequency substantially equal to the resonantfrequency of the series LC circuit.